A cat flap typically takes the form of a door pivotally mounted in a frame, the door being of a size such that the cat can readily push the door open. Gravity or a small spring force then returns the open door to its closed position in order to eliminate unwanted draughts.
One drawback of such cat doors that concerns many cat owners is that stray cats will often try to enter the house. In order to prevent this different forms of "selective" cat doors have been designed. Such designs ensure that the door is locked into the house by a latch that is rendered inoperative in response to an "operator" carried by the cat. A review of known solutions and their associated drawbacks will now be given:
British Patent No. 1567001 presents a solution utilising a magnetic responder linked to a latch member that is attracted towards an operator made of soft iron or to a bar magnet attached to the cat's collar. The principal drawback of this arrangement is that the responder and latch member have a very high inertia and will not always respond quickly enough to a cat's magnet passing over the magnetic responder. Thus the cat may not have deactivated the latch by the time that it presses the door against the latch. From this point on the friction of the latch against the door will prevent operation.
The same Patent also puts forward two other possible ways of producing a selective cat flap, although little detail is given. One solution is provided by using a magnetic switch. Such an arrangement, particularly when executed with a reed switch, overcomes the problem of slow response. It is also very cheap to produce as reed switches are available in quantity at very low cost. However, even with the most sensitive reed switches used in conjunction with the largest magnets a cat can practically carry the cat must be capable of reliably positioning its magnet within an approximately spherical envelope of response extending only 2.5 cm or so from the switch centre. This cannot be relied on in practice as cats will often wish to enter the door from different angles. It is not at all unusual, for instance, for a cat to enter a cat door by pressing at one or other of the sides of the door. Such behaviour could clearly cause a significant problem if there is no means to guide the operating magnet into sufficient proximity to a centrally mounted switch. It might be argued that a very narrow door might overcome the problem, but in practice if the door is constructed for a small cat it would be too narrow for a large cat to enter.
The problems of using a single magnetic switch are largely overcome by the remaining proposal of British Patent No. 1567001, namely by using an inductance loop extending around the opening in conjunction with a control circuit to detect change in inductance. The major problem with this solution lies in the high cost of production. It is also possible that such a system could be incorrectly triggered by, say, a cat wearing a steel address-carrying barrel or even by a buckle on a collar.
British Patent No. 1588673 presents a solution using a magnetically operable switch in conjunction with adjustable biassing means. In this way it is possible to render a single reed switch sufficiently sensitive to allow for the deficiencies of using a single, unbiassed switch. However, in practice such a solution demands relatively expensive circuitry to create a time delay and then energise a resetting coil positioned around the reed switch in order to reopen the reed switch, which could otherwise remain closed under the influence of the biassing magnet. Another disadvantage of this solution is that the adjustable biassing needs adjustment in situ to compensate for local magnetic effects. This can be awkward and is certainly an undesirable task for the cat owner to perform.
A further solution is provided by U.S. Pat. No. 4,022,263 in which the problem of encouraging the cat to enter the door centrally and thus come into sufficient proximity to a single unbiassed magnetically operable switch is achieved by mounting the switch on an external door taking the form of a flexible iris positioned in front of an interior, swinging door. The swinging door is normally retained by a solenoid catch such that when the catch is depressed the door swings out of position under force of gravity and is then free to allow the cat full entry into the house. The swinging door is recocked once the cat has passed through the door (in either direction). In practice it cannot be guaranteed that the cat's passage through the door will ensure that the swing door is recocked. Certainly if the cat passes slowly enough through the door such an action will not occur, thus rendering the flap non-selective. Another disadvantage of this solution relates to the expense of production--largely as a result of the two door construction. A further disadvantage of this system concerns the bulk of the unit, which renders it unsuitable for installation in a domestic door.